The impedances of radiofrequency (RF) signal lines and circuit elements provided for RF signals should be matched to one another in order to minimize undesirable reflections of the signals, for example during the transition from the RF line to a circuit element, or to minimize attenuation of the signal strength.
In mobile communication devices, for example, the impedance of an antenna, which may be variable, should be adapted to the impedance of the circuit component electrically connected thereto, for example a front-end circuit. The impedance of the antenna of a mobile communication device is variable, in particular, when the antenna is a planar antenna whose impedance greatly depends on the spatial distance from other objects or on other external influences.
U.S. Patent Application Publication No. 2008/0274706 A1 discloses an impedance matching circuit of a mobile communication device. The impedances of an antenna and the signal path of the front-end circuit are matched by means of tuning, i.e. means of adaptive matching with the aid of an impedance matching circuit that is electrically connected between a power amplifier and the antenna. The measured transmission power which is determined by a coupler, which is coupled to the signal path, is used as the basis for the adaptive matching, for example. The impedance is continuously matched to the current (load) impedance of the antenna in a control loop.
A problem with known impedance matching circuits is that the adaptive impedance matching, which is carried out in a control loop, also consumes electrical power—for example from a rechargeable battery of a mobile communication device—even when impedance matching is not actually required at all. Impedance matching may be unnecessary if the impedance has been well-matched and the load impedance does not change. One example of such a situation is when the mobile communication device is electrically connected to an external antenna (for example an external antenna on the roof of an automobile). A similar situation occurs when the signal path of the mobile communication device is electrically connected—for example for test purposes—to circuits of an external test environment. Nevertheless, the load impedance may also change over time in such cases.
Another problem with known impedance matching circuits is the complexity of modern multiband/multimode communication devices, in particular those having a plurality of antennas. A regulating system for adaptively matching the impedance of a frequency band generally does not function if it is operated at frequencies in another frequency band. In particular, said system does not function when it is operated in different bands.
Another problem with known impedance matching circuits is that of matching the load impedances of variable-impedance circuit elements when a corresponding device has a plurality of variable-impedance circuit elements, in particular when these circuit elements interact with one another. One example of this situation is a modern communication device in which a plurality of antennas are installed. The plurality of antennas may be provided, for example, in order to serve different frequency bands or to operate in different transmission systems (for example GSM, W-CDMA, etc.). An increased energy requirement is produced, for example, by virtue of the fact that an antenna which is not operating interacts with an operating antenna and interferes with good adaptive matching of the operating antenna.
Impedance matching circuits known from the prior art only match the impedance of antennas which are currently operating. The interaction between an operating antenna and a second antenna which is currently not operating may result in the abovementioned problem that the emission of the active antenna is impaired by interaction with the inactive antenna.
Another problem with known impedance matching circuits is that the emitted RF power is measured as the basis for the impedance matching. However, if the emitted RF power is too low to draw meaningful conclusions on the impedance matching, the impedance matching operation unnecessarily consumes energy, while effective impedance matching is not guaranteed.
In particular, since the trend is moving towards ever smaller, more powerful, and lighter mobile communication devices, there is an interest in having an impedance matching method available which has a reduced energy requirement in comparison with known methods.
Therefore, a need exists for an energy-saving impedance matching method, which accounts for a multiplicity of different situations and carries out impedance matching in an optimum manner despite different situations. Aspects of the present disclosure are directed to satisfying these and other needs.